Congress isn’t planning to tackle climate change anytime soon, which means the Environmental Protection Agency is now the last line of defense. But could the EPA’s new rules on carbon pollution get tossed out by the courts? We’re about to find out.

To regulate or not to regulate? (David Spencer/Associated Press)

There’s a lot at stake here. A 2010 report from the World Resources Institute found that the EPA’s climate rules, when fully deployed, could cover about three-quarters of the country’s greenhouse-gas sources and reduce U.S. carbon emissions anywhere from 5 percent below 2005 levels by 2020 to 12 percent. (For reference, the Obama administration pledged a 17 percent cut at the Copenhagen climate change conference.) On its own, that can’t solve the world’s climate woes. But at the moment, the EPA is the one agency in the United States that’s seriously trying to address the issue.

If the industry lawsuits succeed, the EPA’s recently agreed-to fuel-economy standards for cars and light trucks could, potentially, get nixed. So could the agency’s forthcoming rules on power plants, which analysts say would make it impossible to build new coal plants in the United States for the foreseeable future. (The EPA is also mulling rules to reduce emissions from existing power plants, although details on these regulations remain murky.)

The plaintiffs challenging the EPA — which include energy, mining, and farming industry groups, as well as individual companies such as Peabody Energy — are pursuing a variety of different arguments to get the rules struck down. Here’s an overview of their strategy:

1) Disputing the science on climate change. First, the plaintiffs will argue that the EPA’s endangerment finding — in which the agency ruled that greenhouse gases “threaten the health and welfare of current and future generations” — was wrongly decided. Some of the challenges before the court are quibbling with climate science, and others argue that the EPA should have done its own original research, rather than basing its determination on outside reports such as those from the Intergovernmental Panel on Climate Change.

If the courts were to strike the endangerment finding down, then the EPA would have no authority to regulate carbon dioxide, period. But legal experts say that scenario is unlikely. “Courts generally give deference to agencies on matters that are in their purview,” says William Bumpers, a lawyer with Baker Botts — which, in this case, would include the EPA’s scientific expertise on public health.

2) Challenging individual regulations. Second, the plaintiffs will try to challenge the EPA’s mobile-source rule, which has already been put into effect. Over the past few years, the EPA has come to an agreement with the Department of Transportation to regulate tailpipe emissions from cars and light trucks by setting stricter fuel-economy standards, raising the corporate average to 54.5 miles per gallon by 2025. By and large, the auto industry supports this move, since the standards would otherwise be left to individual states, creating a messy patchwork of regulations.

For energy companies, however, the main significance of the mobile source rule is that once it came into effect, greenhouse gases officially became a regulated pollutant under the Clean Air Act. That means that anyone who wants to build a large new power plant or factory that emits carbon has to go through the permitting process under the Clean Air Act and show that they’ll use the “best available control technology” on greenhouse-gas pollution. If courts struck down the mobile source rule, then these requirements could be delayed temporarily.

3) Creating total chaos by killing the “tailoring rule.” The plantiffs’ final tactic is to try to force a political crisis. Here’s how this would work. The EPA has proposed a “tailoring rule” to make sure that the permitting program applies only to the very largest polluters (emitting more than 100,000 tons of carbon dioxide per year). Without this rule, some 6 million facilities across the country might have to apply for permits, including some buildings that burn heating oil, small businesses, churches and so forth. State regulators would be overwhelmed. It’d be chaos, in other words.

The trouble with this strategy is that the tailoring rule is a rather significant revision to the original Clean Air Act, which is pretty explicit that the thresholds for regulation should be much lower. So industry groups are challenging the rule’s validity. Essentially, the plantiffs would prefer that the EPA regulations hit everyone and bring about an apocalyptic scenario, putting pressure on Congress and the Obama administration to scuttle the climate rules entirely.

Legal analysts say that the tailoring rule is the argument to watch here — the dispute that could make or break the EPA’s climate rules. Not surprisingly, both environmental and industry groups are confidently predicting victory.